Heater



FIG. 1 is a perspective view of the ornamental design for a heater.

FIG. 2 is a side view of the ornamental design for a heater; and,

FIG. 3 is an opposite side view of the ornamental design for a heater.

The broken lines showing in the Figures are environmental and form no part of the claimed design. 

The ornamental design for a heater, as shown and described. 